A scanning device, which can be used according to the aforementioned DE 603 02 183 T2, for example, in a laser printer or, according to the present invention, for example, for the direct exposure of printed circuit boards, scans a flat target surface with a light beam along a scan line in a scan direction, while the target surface is moved to the scanning device or said scanning device is moved to the target surface in a feed direction that runs perpendicular to the scanning device.
The light beam scans in a scan direction by means of a scanning unit, for example, with a galvanometer mirror or a polygon mirror, which rotates at a uniform angular velocity. In order for the light beam to scan the target surface at a uniform velocity, a so-called f-theta lens is used, according to the aforementioned DE 603 02 183 T2, as an optical system that is downstream of the scanning unit (post-scanning optical system). Such an f-theta lens has a distortion characteristic, which in the ideal case satisfies y=f*θ (f=focal length, θ=coupling-in angle=angle that the axial ray of a light beam entering the f-theta lens encloses with the optical axis of the f-theta lens; y=image height).
Generically, a scanning device includes a light source; an optical system (pre-scanning optical system), which is upstream of the aforementioned scanning unit, for shaping and guiding a light beam, coming from the light source, onto the scanning unit; and an optical system (post-scanning optical system), which is downstream of the scanning unit, for shaping and guiding the light beam, deflected by the scanning unit, onto a target surface.
An ideal imaging of the light beam at the image height Y in the plane of the target surface, i.e. in the scan direction at a distance y from the optical axis of the f-theta lens (hereinafter the ideal image position) is performed, provided that the optical system, formed by the scanning device, is monochromatic, only for a light beam with one wavelength. This means that for light beams having a wavelength spectrum that has not only a single wavelength within a negligible bandwidth, wavelength-dependent images are produced in multiple image positions or, in the event of a wide bandwidth, an image, which is expanded in the depth and in the cross section, is produced around the ideal image position. In a typical example for a monochromatic system, all of the refractive elements can be made of the same material.
This may apply, for example, when the wavelength of a light beam changes at the working temperature. That is, its working wavelength is in a wavelength range between two wavelengths, for example, as stated in the aforementioned DE 603 02 183 T2, between 400 nm and 410 nm.
This can also apply, for example, when the light beam has beam components of two different wavelengths, such as, for example, 375 nm and 405 nm; or the light source emits a light beam having a broadband between two wavelengths.
In order to be able to obtain a high resolution image of such light beams having beam components of two wavelengths in an ideal image position and, as a result, the image quality is improved on the whole, the optical system has to be achromatic. That is, it is calculated in such a way that it is corrected for these two wavelengths.